Industrial GFCIs are finally here

Will they help make your plant and workers safer?

By Tony Locker and Nehad El-Sherif, Littelfuse June 11, 2013

Electric shock has always been a vital concern for plant maintenance workers. A recent IEEE white paper points out that from 1992-2002, there were 3,378 fatal electrical injuries listed in the Census of Fatal Occupational Injuries, and all but 30 were attributed to electrocution.

Anywhere plant equipment and workers interact—especially if water or moisture is involved—there is a risk of ground faults, which can injure or kill workers. As little as 50 mA can stop a human heart (or even lower in the worst of conditions).

That’s why the NEC requires ground-fault circuit interrupters (GFCIs) in homes wherever people, water, and electricity may be present (such as a bathroom). In such applications, a UL-listed device is required. However, residential GFCI devices are impractical for industrial facilities as they would cause too many nuisance trips. That’s why, until now, industry has not made extensive use of these people protection devices at higher voltages. Other devices—ground-fault relays (GFRs), for example—are frequently used, but they are intended to protect equipment, not people, and to avoid nuisance tripping their thresholds are set too high to protect workers.

Advances in GFCI technology now make it possible to provide sensitive, low-level protection without nuisance tripping at higher voltages (See Figure 1). Last year UL approved the first GFCI for applications up to 600 V, and some experts feel it’s only a matter of time before GFCIs are required by NEC and OSHA to protect industrial workers.

This article will explain these new devices, the regulations that govern their use, and where they’re best applied.

What a GFCI does

A ground-fault circuit interrupter, whether for industrial or household use, constantly monitors for dangerous ground current by comparing current levels on outgoing (hot) and return conductors; any current flowing through a person goes directly to ground, bypassing the return conductor. When the GFCI detects a current sufficient to injure a person, it instantly shuts off the power by physically interrupting the circuit.

GFCIs for residential use trip at 6 mA and up to 240 V, per UL943C Class A. Unfortunately, these units cannot be used in industrial facilities, which have higher voltage, load current, and leakage to earth. A Class A GFCI will nuisance trip unacceptably in an industrial facility.

Industrial regulations and standards

Until recently there was no UL listing for GFCIs designed for higher power use. In 2000, UL addressed GFCIs for higher voltage applications with a draft standard “Outline of Investigation” called 943C and added to it in 2009, but no commercially available product could pass its test requirements until recently. UL 943C Class C, Class D, and Class E define the characteristics expected of a GFCI operating up to 600 V.

UL 943C sets the trip threshold of special purpose GFCIs at 20 mA—low enough to provide worker shock protection but high enough in most applications to avoid nuisance tripping.

Although industrial GFCIs have higher current trip settings than residential GFCIs, they keep workers safe because of their fast reaction time. Like household devices, their response to ground current follows an inverse-time curve (See Figure 2); a current of 20 mA will cause a trip in about 1 sec (quickly enough to prevent injury at this current level), and higher currents will cause a trip within 20 ms.

Because of the wide range of industrial environments, 20 mA may not be the ideal trip threshold in all applications and units with adjustable trip setting may be needed. But because GFCIs are defined as devices having a fixed trip setting, devices with selectable trip levels cannot be called GFCIs; rather, they are called equipment ground-fault protection devices (EGFPDs). An EGFPD may offer trip levels that can be set from 6 to 50 mA.

The higher mA settings would be useful in applications where the 20 mA setting is causing nuisance tripping, and a lower mA would be useful when there is a need for additional safety. EGFPDs also offer flexibility for global companies wishing to standardize on one device across regions that have different trip level standards. Although EGFPDs can be used for people protection, UL lists them in a category for equipment protection rather than for people protection.

EGFPDs and GFCIs differ from the GFRs familiar to most maintenance managers. GFRs are intended to protect equipment in grounded systems, not people, and they do not open the affected circuit themselves, but send a signal upstream to a breaker. While the GFR reacts in 8 to 10 ms, the upstream circuit breaker may take 30 to 50 ms to open—enough time for an electrical shock to stop someone’s heart. A GFCI or EGFPD contains its own interrupting device (a relay, contactor, or circuit breaker) to interrupt power, and it interrupts power much more quickly. Per UL 943C, a GFCI must open the circuit to interrupt power in less than 20 ms for higher currents. This time is fast enough to prevent shock injury.

UL wanted the UL 943C standard to ensure a high level of safety and added the requirement that the GFCI device must monitor the ground wire for open connections, so that if the connection to ground is lost, then the device will open the circuit. Ground wire monitoring ensures that voltage on the equipment chassis is not elevated relative to ground when an internal ground-fault occurs at the load. The device cannot be reset until the connection to ground is restored. This is an important safety feature that is often overlooked until it is too late.

At present the National Electrical Code (NEC) does not require broad use of industrial GFCIs because of the perceived unreliability of previous products, but it does recommend the use of other protected devices and GFCIs for specific applications. As awareness of UL 943C-listed products increases, acceptance of GFCIs will also increase, and experts expect the NEC to be updated to require GFCI protection in more locations.

GFCIs for lower voltage applications (up to 240 V) have been on the market for a long time, but their use has not always been practical in industrial applications. GFCIs for higher voltage applications (480-600 V) did not exist until last year. Now that industrial GFCIs (See Figure 3) are available for equipment operating at this level of voltage, and are UL listed, the adoption of such protection is expected to grow.

How to apply industrial GFCIs

Industrial GFCIs can be used on equipment subject to washdown cleaning, process equipment that handles wet material, such as large pumps, mixers, wet saws; equipment that comes into frequent contact with workers, such as arc welding stations; and portable electric equipment used outdoors, where long power cords and less robust temporary connections may be exposed to rain and moisture.

Industrial GFCIs may be integrated by the equipment OEM or panel builder, or they may be installed in an electrical cabinet such as a motor control center. They are also available with their own enclosures, for mounting to the side of a machine or a wall.

The wiring is simple: attach source power on one side and the load on the other side (See Figure 4). No wiring to a circuit breaker is required because the GFCI internal interrupting mechanism can interrupt power on its own. In addition, there may be signal wiring for optional alarm communication.

Some models are equipped with built-in overcurrent protection (fuse), which allows for a high interrupting rating (50 kA in some cases). This protects the internal contactor from damage and doesn’t require the user to install a current limiting device upstream of the industrial GFCI. Some models also offer undervoltage, brownout, and chatter detection.

GFCI Standards and Classes

UL Standards

  • UL 943 – This Standard applies to Class A, single- and three-phase GFCIs intended for protection of personnel, for use only in grounded neutral systems in accordance with the National Electrical Code (NEC), ANSI/NFPA 70, the Canadian Electrical Code, C22.1 (CEC), and Electrical Installations (Use), NOM-001-SEDE. These devices are intended for use on alternating current (AC) circuits of 120 V, 208Y/120 V, 120/240 V, 127 V, or 220Y/127 V, 60 Hz circuits.

  • UL943C – These requirements cover ground-fault circuit interrupters intended for use in one of the following applications:

    • Where the voltage to ground is greater than 150 V and equipment grounding or double insulation is required by the National Electrical Code or ANSI/NFPA 70.

    • Where the voltage to ground is 150 V or less and equipment grounding or double-insulation is provided, but the use of a Class A ground-fault circuit interrupter is not practical.

  • UL 1053 and UL 943 both apply to EGFPDs.

GFCI Classes

  • CLASS A – A GFCI that will interrupt the circuit to the load when the ground-fault current is 6 mA or more but not when the ground-fault current is 4 mA or less.

  • CLASS B – A GFCI that will interrupt the circuit to the load when the ground-fault current exceeds 20 mA. Used only with swimming pool underwater lighting fixtures installed prior to local adoption of the 1965 edition of the NEC.

  • CLASS C – A GFCI that is intended to be used in circuits with no conductor over 300 VAC to ground where reliable equipment grounding or double insulation is provided.

  • CLASS D – A GFCI that is intended to be used in circuits with one or more conductors over 300 V to ground, and with specially sized, reliable grounding, to provide a low impedance path so that the voltage across the body during a fault does not exceed 150 V.

  • CLASS E – A GFCI that is intended to be used in circuits with one or more conductors over 300 V to ground but with conventional equipment grounding provided for the protected equipment in the system or double insulation. These ground-fault circuit interrupters respond rapidly to open the circuit before the magnitude and duration of the current flowing through the body reach the threshold for ventricular fibrillation.

Conclusion

Ground-fault circuit interrupters for industrial use have finally become available. One hopes that their wide adoption will help to reduce the number of electrical shock fatalities by a substantial number.

Notes

1. IEEE White Paper, “The Effects of System Grounding, Bus Insulation and Probability on Arc Flash Hazard Reduction–the Missing Links,” by Nelson, Billman, and Bowen, 2012.

See articles on electrical safety and arc flash below.